Due to the giant strides of electrical industry processing, the demand for the electrical product efficiency is getting more and more. As function of the product is getting variety; the circuit design is getting complication. However, when a precise and complicated circuit is operated within high supply voltage, such not only causes high power consumption and affects the response time, but also reduces the yield of the semiconductor process for the circuit manufacturing.
In accordance with the general laser diode control circuit loop as the prior art, the output driving voltage should be as higher voltage operation to ensure that the laser diode driving circuit works in a normal operation status. The device as shown on FIG. 1, comprises a high speed switch unit 13, a third transistor 135, a laser diode 11, and a load resistor 17. The high speed switch unit 13 further comprises a first transistor 131 and a second transistor 133. And, the first transistor 131 and the second transistor 133 are respectively as a NPN bipolar transistor.
The collector end of the first transistor 131 connects with a supply voltage 13 through the load resistor 17. The collector end of the second transistor 133 connects to the laser diode 11, and the laser diode 11 connects with the supply voltage 12. The base end of the first transistor 131 can receive a first voltage 141, and the base end of the second transistor 133 can receive a second voltage 143. The emitter end of the first transistor 131 and the second transistor 133 connect with the collector end of the third transistor 135, and a rear current is generated from the third transistor 135.
The emitter end of the third transistor 135 connects to the ground, and the base end thereof receives an input bias 155, which supplies the third transistor 135 works in the operation area.
The prior art laser diode driving circuit 10 works within high voltage operation by supply voltage 12 (such as 5V). The supply voltage 12 subtracts the critical voltage of the laser diode 12 (such as 1.8V), the output driving voltage of the output driving voltage end 19 is 3.2V, such supplies enough for the high speed switch unit 13 and the third transistor 135 working in the operation area. Thus, when the supply voltage 12 is 5V, the laser diode driving circuit 10 can be worked regularly.
When the supply voltage 12 is as low voltage (such as 3.3V), the supply voltage 12 subtracts the critical voltage of the laser diode (1.8V), the output driving voltage of the output driving voltage end 19 is only 1.5V. Referring to the conducting wire of the circuit, the impedance factor of the series resistance, and the individual critical voltage Vbe of the high speed switch unit 13 and the third transistor 135, the output driving voltage must be larger than 1.8V to ensure that the high speed switch unit 13 and the third transistor 135 can be worked in operation area. Obviously, the supply voltage 12 is insufficient, which causes the failure operation for the laser diode driving circuit 10.
However, the low power consumption is the mainstream product nowadays. The supply voltage 12 of many electrical products is getting to be used as low voltage (such as 3.3V) from prior high voltage (such as 5V). Therefore, the laser diode driving circuit 10 works within high voltage supply should not be suitable for the demand of the recent circuit design.